Control system for sodium-sulfur battery

ABSTRACT

A control system for a sodium-sulfur battery, which is composed of a plurality of battery modules connected in series, includes a control device having at least a temperature measuring unit for measuring a temperature of each battery module, a voltage measuring unit for measuring a voltage thereof, and a current measuring unit for measuring a current thereof assembled as a single control device. Preferably, the system includes a unit for detecting the end of discharge and the end of charge. Thus, the time lag in the detection of the end of discharge and the end of charge may be prevented. The reliability of a NaS battery during long-term operation is improved. A fluctuation in the power consumption of heaters during the driving of the NaS battery is reduced. The space required for installing power equipment such as a transformer may be omitted.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to a control system for asodium-sulfur battery, which has a function capable of controllingproperly charge and discharge of a sodium-sulfur battery, adjusting atemperature thereof and the like, and a sodium-sulfur battery beingprovided with such a control system.

[0003] 2. Description of the Related Art

[0004] A sodium-sulfur battery (hereinafter, sometimes referred to as aNaS battery) comprises a plurality of sodium-sulfur cells connected witheach other, and is being put into practical use for variousapplications. Such applications include, for example, the use of a powerstorage system for leveling electric power demand so as to cope with alarge difference in demands during daytime and nighttime, particularly,a peak-cut power supply system for supplying electric power for timeperiods during which power demand sharply increases in the summerseason, or an emergency power supply system in natural disastersituations.

[0005] The NaS battery is used, for example, as a NaS battery powerstorage system in which a circuit is formed between a NaS battery, anAC/DA converter, and other components; said NaS battery beingconstituted from the components produced in the manner mentioned below:Firstly, a NaS battery string (a group of cells) is formed by connectinga plurality of cells in series, then a NaS battery block is formed byconnecting a plurality of thus formed NaS battery strings parallel toeach other. Thereafter, a plurality of a plurality of thus formed NaSbattery blocks are connected in series to form a NaS battery module(hereinbelow, sometimes simply referred to as a battery module), then aNaS battery is formed by connecting a plurality of thus formed NaSbattery modules in series.

[0006] The NaS battery is a secondary battery in which molten metalsodium as a cathode active material and molten sulfur as an anode activematerial are arranged separately from each other using a β-alumina solidelectrolyte having selective permeability toward sodium ions. Thedischarge of the NaS battery is done by the following manner. Moltensodium liberates an electron, and becomes a sodium ion. Thus formedsodium ion moves toward the positive electrode by passing through saidsolid electrolyte and then reacts with sulfur and electron supplied froman external circuit to produce sodium polysulfide. On the other hand,the charge is done as a reverse process of the discharge; that is, asodium and sulfur is formed as a result of reaction of sodiumpolysulfide with emission of an electron. In the viewpoint ofcharge/discharge efficiencies, preferably, the NaS battery is operatedat a high temperature of 280° C. or more in consideration of thetemperature characteristics of sodium ion conductivity with respect toβ-alumina. However, the operating temperature of the NaS battery islimited due to the heat resistances of various components constitutingthe battery, and the like. Therefore, it is important to make the NaSbattery charge or discharge with keeping an operating temperature withina predetermined range of from 280 to 360° C., for example.

[0007] Further, it is also important to properly control the chargingand discharging operations of the NaS battery. For example, in eachcell, an open circuit voltage at the depth of full-charge is keptconstant to 2.075 V. As the cell is being discharged, an electromotivevoltage gradually decreases. When the discharge is completed (the end ofdischarge), the open circuit voltage is set to substantially 1.82 V.However, for example, a voltage measured during the discharge is lowerthan the open circuit voltage by the product (voltage drop) of internalresistance and a discharge current. Therefore, during the operation ofthe NaS battery, it is necessary to compensate the voltage drop byadding a voltage equal to voltage drop to a measured voltage to obtainan open circuit voltage during the discharge and then detect the end ofdischarge.

[0008] A conventional control system, which has been used to implementthe above proper operation of the NaS battery, comprises individualmodule control devices (hereinbelow, sometimes referred to as a modulecontroller), each of which is provided for each of the battery moduleslaid in a frame for NaS battery, and a general-purpose control devicesuch as a sequencer installed on a control panel provided independentlyof the frame for NaS battery. Each module controller measures a voltageand a temperature of each battery module to monitor the operating statethereof and also turns a heater provided for each battery module on oroff to adjust the operating temperature of the NaS battery. For example,a discharge current of the NaS battery is measured using a currentmeasuring function of the sequencer serving as the general-purposecontrol device. In the sequencer, a voltage drop is calculated to obtaina discharge cutoff voltage. Thus, the end of discharge in the NaSbattery is detected. The cutoff voltage means a reference voltage usedto determine the end of charge or discharge of the NaS battery.

[0009] An illustrative showing of a conventional control system for aNaS battery is given in FIG. 2 in which five battery modules areconnected in series. The control system comprises module controllers 26,each of which is provided for each NaS battery module 24 in a batteryframe 21 of a NaS battery 22, and a sequencer 23 provided for a controlpanel 31 arranged independently from the battery frame 21 of the NaSbattery 22. Each module controller 26 has temperature measuring elementand voltage measuring element, turns a heater 25, to which heater power27 is supplied through a heater power supply line 127, on or off inaccordance with the measured temperature to control the operatingtemperature of the NaS battery 22. Each module controller 26 also hastransmitting/receiving element according to RS-422 standard or the liketo transmit measurement data and a signal indicative of the operatingstate of the corresponding heater 25 to a control unit 28 of thesequencer 23. The sequencer 23 receives information indicating the stateof the battery each such as a temperature and a voltage from each modulecontroller 26 through the transmitting/receiving element according toRS-422 standard. The sequencer 23 includes a measurement unit 29 formeasuring an output current (discharge current) of the NaS battery 22comprising the battery modules 24 connected in series. These measurementvalues and information are displayed on a display (not shown) providedfor the control panel 31 and are also generated as external signals 20.The external signals 20 can be confirmed by a remote monitoring hardwarethrough, for example, a data link.

[0010] The above conventional control system has sufficiently served forthe purpose of controlling the NaS battery in the development stage forpractical application. However, in the current situation, the NaSbattery is being widespread. The improvement in long-term reliability, areduction in the trial operating cost of equipment serving as, forexample, a power storage system, and a reduction in time required fordesign and manufacture are further desired in the market. Then, thefollowing problems come to exist.

[0011] (a) Misdetection at the End of Charge/Discharge

[0012] Hitherto, for example, when the end of discharge is detected, adischarge cutoff voltage V_(L) is obtained by the following expression(1) using a discharge current Id measured by a current measurement unitof a sequencer, internal resistance R of each battery module, and atemperature coefficient K_(t), which is fluctuated by an operatingtemperature T:

V _(L) =V _(O) ×n−I _(d) ×R×K _(t)  (1).

[0013] The discharge cutoff voltage V_(L) is compared to an actualoperating voltage V of each battery module, the actual operating voltageV being measured by each module controller and being then transmitted tothe sequencer. When the following expression (2) is satisfied, it wasjudged to be the end of discharge:

V_(L)>V  (2).

[0014] In this instance, reference symbol V_(O) denotes an open circuitvoltage of each cell just before sodium at the negative electrodebecomes short, and this open circuit voltage V_(O) is usually about 1.82V. Reference symbol n denotes the number of cells included in eachbattery module. In other words, the discharge cutoff voltage V_(L)indicates an operating voltage at the theoretical end of discharge ofthe NaS battery. However, since data indicating the operating voltage Vis transmitted from the module controller to the sequencer and thecomparison and the determination are then performed, a transmissiondelay occurs. Accordingly, the operating voltage V is not accuratelysynchronized with the discharge voltage I_(d), a delay between thedischarge cutoff voltage V_(L) calculated from the discharge currentI_(d) and the operating voltage V to be compared thereto is generated.Thus, some time lag always exists in the determination of the end ofdischarge. Consequently, charged power cannot be effectively used.

[0015] When the end of charge is detected, a charge cutoff voltage V_(H)is obtained by the following expression (3) using a charge current I_(c)measured by the current measurement unit of the sequencer and theinternal resistance R of each battery module:

V=(V _(I)+α)×n−I _(c) ×R  (3).

[0016] The charge cutoff voltage V_(H) is compared to the actualoperating voltage V of each battery module, the voltage V being measuredby each module controller and being then transmitted to the sequencer.When the following expression (4) is satisfied, it was judged to be theend of charge:

V_(H)<V  (4).

[0017] In this instance, reference symbol V_(I) denotes an open circuitvoltage of each cell at the end of charge, and this open circuit voltageV_(I) is generally 2.075 V. Reference symbol n denotes the number ofcells included in each battery block. Reference symbol a denotespolarization resistance generated at the end of charge. Thispolarization resistance is generally 0.05 to 0.15 V. In other words, thecharge cutoff voltage V_(H) indicates a voltage obtained by allowing thepolarization in addition to the open circuit voltage at the theoreticalend of charge of the NaS battery. However, due to a transmission delayin the same way as the case of discharging, the operating voltage V isnot accurately synchronized with the charge current I_(c). Thus, thereis observed a delay between the charge cutoff voltage V_(H) calculatedfrom the charge current I_(c) and the operating voltage V to be comparedthereto. Thus, some time lag exists in the determination of the end ofcharge. Accordingly, charging is not sufficient and the capacity of thebattery cannot be effectively used.

[0018] (b) Fluctuation in Power Consumption of Heaters

[0019] A three-phase three-wire AC power supply is used as a powersupply for heaters. The heaters, provided for the respective batterymodules, are connected to each other in balance so that each heaterserves as a rated line load between two lines. For example, 10 kW isapplied as a load between R-phase and S-phase, 9 kW is applied as a loadbetween S-phase and T-phase, and 10 kW is applied as a load betweenT-phase and R-phase. Inherently, the heaters serve as temperatureraising elements for setting the operating temperature of the NaSbattery in a proper range. The heaters are individually turned on or offin accordance with the state of the NaS battery. Each conventionalmodule controller turns each heater on or off independently of the othermodule controllers. Accordingly, all of the heaters are simultaneouslyturned on or off at a certain probability. Since a fluctuation in thepower consumption of the heaters is very large, each of a transformerand a circuit breaker needs a capacity corresponding to those of theheaters. Further, the voltage fluctuates due to changes of the heaterloads.

SUMMARY OF THE INVENTION

[0020] The present invention is made in consideration of theabove-mentioned problems. It is an object of the present invention toprevent misdetection of the end of charge and the end of discharge toincrease the long-term reliability of a NaS battery. It is anotherobject of the present invention to reduce a fluctuation in the powerconsumption of heaters during the operation of a NaS battery in order toeliminate a fluctuation in voltage, and to omit the space required forproviding power receiving equipment such as a transformer in order toreduce the cost for the control system.

[0021] To solve the problems, investigations have been repeatedlyperformed and studies have made progress. As a result, it has been foundthat the above objects can be accomplished by employing a system inwhich all elements for controlling temperatures, voltages, and currentsof a predetermined number of battery modules are assembled as a singlecontrol device. In addition, the reduction in the cost of initialequipment of a NaS battery serving as, for example, a power storagesystem, and a reduction in time required for design and manufacturethereof are expected by employing this system.

[0022] In other words, according to the present invention, there isprovided a control system for a sodium-sulfur battery having a pluralityof battery modules connected in series or in parallel to each other,wherein there is provided a control device housing at least atemperature measuring unit for measuring a temperature of apredetermined plural number of battery modules, a voltage measuring unitfor measuring a voltage thereof, and a current measuring unit formeasuring a current thereof in a proper DC line for assembling as asingle united system. Preferably, the system includes a unit fordetecting the end of discharge and the end of charge.

[0023] When the control system is used in a sodium-battery in which eachbattery module has a heater, a three-phase three-wire power supply isused to supply power to the heaters, and the respective heaters areconnected so that each heater serves as a line load between lines, it ispreferable that the control system is provided with a heater controlunit capable of controlling the respective heaters so as to level thepower consumption of the respective lines per unit time.

[0024] Preferably, the sodium-sulfur battery control system according tothe present invention is arranged at the bottom of a sodium-sulfurbattery frame on which a predetermined number of the battery modules islaid. A sodium-sulfur battery having such control system as thatmentioned above is preferably used as a power storage system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a system diagram showing the configuration of a NaSbattery control system according to the present invention.

[0026]FIG. 2 is a system diagram showing the configuration of aconventional NaS battery control system.

[0027]FIG. 3 shows an illustrative example of time schedule of heatersincluded in the control system for NaS battery according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] An embodiment according to the present invention will now bedescribed hereinbelow. A sodium-sulfur battery control system(hereinbelow, sometimes simply referred to as a control system)according to the present invention is a control system for asodium-sulfur battery comprising a plurality of NaS battery modules inseries or in parallel to each other. The upper limit of the capacity ofthe NaS battery, namely, the upper limit of the number of NaS batterymodules is not considered to be limitative. It is considered that thepresent control system can cope with a normal-scale NaS battery, if theupper limit of the capacity of a Nas battery is set to 500 kW; that is,such a capacity attainable by mounting 10 NaS battery modules eachhaving a capacity of 50 kW.

[0029] The control system according to the present invention has twoaspects, namely, first aspect is directed to the management of thecharging and discharging operations of a NaS battery, and second one isdirected to the adjustment of actual loads on a heater power supply.

[0030] According to the first aspect, the characteristic of the controlsystem of the present invention lies in the point that at leasttemperature measuring unit or element for measuring a temperature ofeach battery module of the predetermined number of the battery modules,voltage measuring unit or element for measuring a voltage thereof, andcurrent measuring unit or element for measuring a current thereof arehoused in a single control system. In other words, in the presentcontrol system, a single control device being provided in everypredetermined plural number of NaS battery modules can directly collect,without via transmitting means, temperature data, voltage data, andcurrent data by virtue of a temperature measuring unit or element, avoltage measuring unit or element, and the current measuring unit orelement for the respective modules. It is preferable that a temperatureand a voltage can be measured individually based on each NaS batterymodule and a current can be measured as an output current(charge/discharge current) of the NaS battery module comprising batterymodules connected in series.

[0031] Since a single control device may directly obtain temperaturedata, voltage data, and current data without transmitting means, the endof discharge and the end of charge may be determined without a timedelay so long as the system includes an element or unit for detectingthe end of discharge and the end of charge.

[0032] For example, in the case of detecting the end of discharge, anoperating temperature T, an operating voltage V, and an dischargecurrent I_(d) may be accurately synchronized with each other, takingoperating temperature T of each battery module as a temperature,operating voltage V thereof as a voltage, and discharge current I_(d)thereof as a current. This is because transmitting means is not used inthe present system. Accordingly, as long as the system is provided withan end-of-discharge detecting element or unit capable of determining theend of discharge by comparing the discharge cutoff voltage V_(L) servingas a reference voltage used to determine the end of discharge andobtained by subtracting a voltage equal to a voltage drop, which isobtained on the basis of the temperature coefficient K_(t) corrected bythe operating temperature T, the internal resistance R of each batterymodule, and the discharge current I_(d), from the open circuit voltage(V_(O)×n) at the theoretical end of discharge as shown in the foregoingexpression (1), and then comparing the discharge cutoff voltage V_(L)with the actual operating voltage V according to the foregoingexpression (2), the end of discharge may be more accurately determinedwithout causing a time delay between the discharge cutoff voltage V_(L),which is obtained from the operating temperature T and the dischargecurrent I_(d), and the operating voltage V. Thus, charged power may beeffectively used. There is no problem that the battery cannot bedischarged and charged any longer due to the deficiency of sodium at thenegative electrode caused by over discharge. Therefore, the reliabilityduring a long-term operation of the NaS battery may be remarkablyimproved.

[0033] In the case of the second aspect, the feature of the presentcontrol system lies in the point that the system is provided with aheater control element or unit capable of controlling each of theheaters in such a manner that power consumption between the respectivelines is leveled on a time basis, in the case that a heater is providedfor each of battery modules constituting the NaS battery, a three-phasethree-wire power supply is used, and the heaters are connected so as toserve as loads between lines. The expression “leveling the line powerconsumption on a time basis” means averaging power consumption of theheaters per unit time by staggering times, at each of which thecorresponding heater is turned on, among the lines in order to avoid thesimultaneous turn-on of all the heaters. Thus, the capacity of auxiliaryequipment can be reduced, resulting in a reduction in the cost ofequipment.

[0034] As mentioned above, according to the first aspect, a singlecontrol device includes a temperature measuring element or unit formeasuring a temperature of each of the predetermined plural number ofthe battery modules. Consequently, the temperatures of all the batterymodules may be simultaneously grasped. Therefore, there is no necessityof turning on or off of the corresponding heater in accordance with theresult of the operating-temperature measurement of the correspondingbattery module after the heaters have been connected in balance so thateach heater serves as a rated load between two lines of the three-phasethree-line power supply. That is, in the case of the present controlsystem, firstly, one may set a time schedule so that the operatingtemperature of each battery module is set in a predetermined range andthe power consumption of the respective lines per unit time is leveled,then, each heater is turned on or off based on this schedule.Accordingly, a fluctuation in the power consumption may be reduced.Resultantly, a fluctuation in voltage may also be reduced and the spacerequired for housing a power receiving equipment such as a transformermay be omitted, as well.

[0035] Further, according to the present invention, the conventionalcombination of control devices provided for respective battery modulesand a general-purpose sequencer is not employed. According to thepresent invention, a control system is equipped with a single controldevice capable of controlling a plural number of battery modules housedin a battery frame for a NaS battery. Accordingly, the present systemmay be manufactured in the form of a compact control system. That is,the present control system may be housed in the battery frame for theNaS battery without using a separate control panel, resulting in areduction in the cost of initial equipment. In this case, preferably,the control system is placed at the bottom of the battery frame. This isbecause a space at the bottom of the battery frame is kept at a lowtemperature of about 40° C. at the maximum and a low humidity of about70% at the maximum. Accordingly, the bottom of the battery frame ispreferably used as a space for placing the control system equipped witha large number of electronic devices.

[0036] Since a separate control panel is not needed, a cost required fordesign and manufacture of the control panel can be omitted. In addition,time required before the delivery of the NaS battery can be shorteneddue to thus eliminated works for the design and the manufacture of thecontrol panel. Therefore, it is expected that the present control systemwould make the NaS battery easily accepted in the market, and,resultantly, this would facilitate the prevalence of the NaS battery.

[0037] The present invention will now be described in detail withreference to the drawings.

[0038]FIG. 1 is an illustrative system diagram showing a NaS batterycontrol system according to an embodiment of the present invention. ANaS battery 12 may comprise five NaS battery modules 14, for example. Acontrol system is placed in a battery frame 11 for the NaS battery 12.The control system comprises a control device 13 comprising ameasurement control unit 19 and a heater driving unit 18. In the controldevice 13, the measurement control unit 19 comprises arithmeticoperating unit, temperature measuring means for measuring the operatingtemperature T of each NaS battery module 14, voltage measuring means formeasuring the operating voltage V thereof, signal output element, andnetwork means. The measurement control unit 19 further includes currentmeasuring unit or element for measuring a current of the NaS battery 12comprising the NaS battery modules 14 in series.

[0039] The arithmetic operating unit may include, for example, a CPU, amemory associated therewith, and an input/output IC for transmittingdata to and receiving data from the other means. The signal outputelement may include, for example, a voltage relay (contact) or ano-voltage relay. One may use RS 422, RS 485, DeviceNet, FLnet, EtherNetor the like as a network means.

[0040] The temperature measuring unit or element may be any device,which may include, for example, one using a thermocouple or a change inelectric resistance based on temperature. Preferably, the temperaturemeasuring unit or element can measure a temperature of a portion of eachbattery module, the portion corresponding to each heater. In otherwords, when a heater corresponding to each battery module is composed ofa bottom beater segment and a side heater segment as will be describedbelow, it is preferable that a temperature at the bottom of each batterymodule and a temperature at the side thereof be measured.

[0041] Preferably, the voltage measuring unit or element can measure avoltage of each battery block in each battery module. This is becausethe accurate measurement can surely avoid over charge or over discharge.Preferably, the current measuring unit or element can measure a currentof the NaS battery 12.

[0042] The heater driving unit 18 includes relays each having a capacitywhich can withstand a current flowing through each heater (load) of,generally, several kW. Each relay comprises, for example, asemiconductor device. Each relay may connect or disconnect each heaterpower supply line 117 so that heater power 17 can be supplied to eachheater 15 or can be stopped. Preferably, a fuse is connected to eachrelay in series in order to protect devices and lines when a shortcircuit occurs in the heater.

[0043] In the measurement control unit 19, measurement values (data) ofa temperature, a voltage, and a current measured by the temperaturemeasuring unit, the voltage measuring unit, and the current measuringunit are received by the arithmetic operating unit. The respectivevalues are also transmitted as external signals 10 through the signaloutput unit or element and the network means.

[0044] In the arithmetic operating unit of the measurement control unit19, during the period of discharging, the discharge cutoff voltage V_(L)is obtained on the basis of the foregoing expression (1) using theoperating temperature T measured by the temperature measuring elementand the discharge current I_(d) measured by the current measuringelement. The discharge cutoff voltage V_(L) is compared to the operatingvoltage V measured by the voltage measuring element. When the foregoingexpression (2) is satisfied, it is judged to be the end of discharge.Thus, discharging the NaS battery 12 is terminated.

[0045] During period of charging, the charge cutoff voltage V_(H) isobtained on the basis of the foregoing expression (3). The charge cutoffvoltage V_(H) is compared to the operating voltage V measured by thevoltage measuring unit. If the foregoing expression (4) is satisfied, itis judged to be the end of charge. Thus, charging the NaS Battery 12 isterminated.

[0046] Prohibiting or stopping charging or discharging is determined onthe basis of the measured temperature, voltage, and current, thusdriving the NaS battery with more safety.

[0047] The measurement control unit 19 allows the signal output unit orelement to generate a heater control signal for each heater to theheater driving unit 18 in accordance with a predetermined schedulestored in, for example, the arithmetic operating unit or element.

[0048] In accordance with the heater control signal (for example, acontact signal) received from the measurement control unit 19, theheater driving unit 18 supplies or stops the heater power 17 to besupplied to each heater 15 through each heater power supply line 117,thereby turning each heater 15 on or off.

[0049]FIG. 3 shows an example of a heater control schedule. According tothe present embodiment, each heater is composed of a bottom heatersegment (5.6 kW) and a side heater segment (1.8 kW) which are not shownin FIG. 1. The bottom heater segment and the side heater segment arecontrollable individually. The heaters are provided for the respectivebattery modules.

[0050] As shown in FIG. 3, each bottom heater segment and each sideheater segment are operated so as to repeat a period CT, in which ONtime and OFF time are equalized, and so as to be ⅙ CT out of phase witheach other. Due to the heater control, the operating temperature of theNaS battery may be kept within the predetermined temperature range andpower consumption between two lines in the three-phase three-line ACpower supply is generally leveled.

[0051] The measurement values of the operating temperature, theoperating voltage, and the discharge current measured by the respectivemeasuring units or elements and a signal indicative of the state (e.g.,the end of discharge) of the NaS battery which can be determined by thearithmetic operating unit or element can be displayed on a display (notshown) provided for the battery frame 11 for the NaS battery 12.Further, a signal indicative of the result obtained by comparing themeasurement values with various set values or fixed values stored in themeasurement control unit 19 through the arithmetic operating unit orelement, for example, an abnormal signal indicative of, e.g., “hightemperature” can also be displayed on the display. These measurementvalues and signals can also be generated as external signals 10. Thesignals can be confirmed in, for example, a remote monitoring hardwarethrough a network.

[0052] The format of the external signals 10 generated by the signaloutput unit or element and the network means is not limited. Forexample, analog signals (currents of 4 to 20 mA DC, voltages of 1 to 5 VDC), digital signals, pulse signals, relay contacts can be used.

[0053] The network connecting the measurement control unit 19 of thecontrol device 13 to, for example, the remote monitoring hardware (notshown) is not limited to the foregoing network means. A network witheasy wiring works to organize the network, higher communication speed,higher noise-resistant properties, and higher resistant properties totemperature changes is preferably used. As a line used to organize anetwork, a coaxial cable or an optical cable is preferably used.

[0054] As mentioned above, according to the present invention, the NaSbattery can be controlled without using a separate control panel(control device). In addition, the erroneous judgement on the end ofdischarge hardly occurs. Further, the respective heaters between therespective lines of the three-phase three-line power supply consumepower in balance. Therefore, for example, a power storage system using aNaS battery including the control system according to the presentinvention exhibits high long-term reliability. Time required for designand manufacture of the power storage system before the delivery thereofcan be easily reduced. The power storage system wherein the presentcontrol system is employed may be made compact, thereby the storagesystem can be easily installed. The cost of initial equipment and thedriving cost can be reduced, resulting in an increase in the demand forthe power storage system.

What is claimed is:
 1. A control system for a sodium-sulfur batterycomprising a plurality of battery modules connected in series or inparallel to each other, wherein there is provided an assembled controldevice as a single control device that comprises at least temperaturemeasuring unit capable of measuring a temperature of each batterymodule, voltage measuring unit for measuring a voltage of each batterymodule, and current measuring unit for measuring a current of eachbattery module, independently among a plural number of battery modulesconstituting a sodium-sulfur battery.
 2. The system according to claim1, which further comprises end-of-discharge detecting unit.
 3. Thesystem according to claim 1, wherein there is further provided a heatercontrolling unit so as to control respective heaters provided to eachbattery module constituting a sodium-sulfur battery in which athree-phase three-wire power supply is used to supply power to theheaters, the respective heaters are connected so that each heater servesas a line load, in such manner that power consumption of the heaters perunit time is leveled.
 4. The system according to claim 2, wherein thereis further provided a heater controlling unit so as to controlrespective heaters provided to each battery module constituting asodium-sulfur battery in which a three-phase three-wire power supply isused to supply power to the heaters, the respective heaters areconnected so that each heater serves as a line load, in such manner thatpower consumption of the heaters per unit time is leveled.
 5. Asodium-sulfur battery power storage system comprising: a plurality ofbattery modules; and a sodium-sulfur battery control system for asodium-sulfur battery comprising a plurality of battery modulesconnected in series or in parallel to each other, wherein there isprovided an assembled control device as a single control device thatcomprises at least temperature measuring unit capable of measuring atemperature of each battery module, voltage measuring unit for measuringa voltage of each battery module, and current measuring unit formeasuring a current of each battery module, independently among a pluralnumber of battery modules constituting a sodium-sulfur battery; thecontrol system being arranged at the bottom of a battery frame.
 6. Thesodium-sulfur battery power storage system according to claim 5, whereinsaid sodium-sulfur battery control system for a sodium-sulfur batteryfurther comprises end-of-discharge detecting unit.
 7. The sodium-sulfurbattery power storage system according to claim 5, wherein saidsodium-sulfur battery control system for a sodium-sulfur battery isfurther provided with a heater controlling unit so as to controlrespective heaters provided to each battery module constituting asodium-sulfur battery in which a three-phase three-wire power supply isused to supply power to the heaters, the respective heaters areconnected so that each heater serves as a line load, in such manner thatpower consumption of the heaters per unit time is leveled.
 8. Thesodium-sulfur battery power storage system according to claim 6, whereinsaid sodium-sulfur battery control system for a sodium-sulfur battery isfurther provided with a heater controlling unit so as to controlrespective heaters provided to each battery module constituting asodium-sulfur battery in which a three-phase three-wire power supply isused to supply power to the heaters, the respective heaters areconnected so that each heater serves as a line load, in such manner thatpower consumption of the heaters per unit time is leveled.